1. Field of the Invention
The present invention relates to a rolling bearing, in particular, a rolling bearing suited to use to rotation supporting part in information machinery such as a hard disc drive apparatus (xe2x80x9cHDDxe2x80x9d hereafter), video tape recorder (xe2x80x9cVTRxe2x80x9d hereafter), or digital audio tape recorder (xe2x80x9cDATxe2x80x9d hereafter), another rolling bearing suited to use to vibration acting part in a swing arm as a part of composing elements of such as HDD, or a further rolling bearing suited to use to machinery requiring quietness as in fan motor, cleaner motor, or turbo charger of vehicle.
2. Description of the Related Art
Generally, in the rolling bearings, rolling movement takes place between bearing rings and rolling elements, and since the bearing rings and the rolling elements receive repeated contacting stress, these members are demanded to be hard, durable to load, long in rolling fatigue life, or excellent in abrasion resistance against sliding.
For these materials, there are generally used SUJ2 of Japanese Industrial Standards as bearing steels, SUS440C or 13Cr martensitic stainless steels of Japanese Industrial Standards as stainless steels, and steel materials, as case hardened steels, equivalent to SCR420 of Japanese Industrial Standards, having passed through quenching, carbinization or carbonitriding treatment. Those are subjected to quenching and tempering for providing required hardness, abrasion resistance and rolling fatigue life, and acquire hardness as HRC58 to 64.
Especially, in rolling bearings to be used to information machinery such as HDD, VTR or machinery requiring quietness as fan motor, requirements to reduction of torque, sound or noises are very severe in addition to excellent abrasion resistance or rolling fatigue life.
For materials of the rolling bearings to be employed to such machinery, very often used are SUJ2 as high carbon Cr bearing steels, SUS440C as martensitic stainless steels, or 0.7C-13Cr stainless steel. Those are subjected to quenching and tempering for providing required hardness and abrasion resistance, and acquire hardness as HRC58 to 64. Excepting special situations, basically the rolling element of the present case is composed of material equal to any one of the bearing rings, outer and inner races.
However, various new problems have nowadays arisen, as transportability is heightened by miniaturization of machinery. That is to say, accompanying with the miniaturization of machinery, chances increase of rolling bearings encountering dropping or vibration during transporting, so that rolling bearings incorporated in machinery are injured though being very small, and it has been apparent that those causes to deteriorate machine performance. In short, in case machinery is effected with shock load, particularly in miniaturized rolling bearings, since a contact area between the bearing ring and the rolling element is small, a raceway surface is permanently deformed in spite of relatively small shock load, causing sound deterioration or unevenness in rotating torque, and the performance of machinery incorporated with rolling bearings is worsened.
This problem is considered to appear because yield stress is low in a soft phase of such as residual austenite existing in steel, and such measures are taken that if being SUJ2, it is subjected to a sub-zero treatment after quenching, or tempering at relatively high temperatures as around 220 to 240xc2x0 C. for lowering the amount of residual austenite and preventing sound deterioration or unevenness in rotating torque by shock load, while keeping the hardness required to rolling bearings.
On the other hand, a main cause of sound deterioration by fine vibration or oscillation of machine is made by injures of rolling elements, and a conventional rolling element composed of steel is, as shown in FIG. 8, formed with considerable fretting abrasion scratches (band pattern of FIG. 8) at contacting part between the inner and outer races. But for fretting created by vibration or movement of machine, a measure by only lubricant has been made, and a measure by improving materials has not been reduced to practice.
With respect to the problem about fretting, a ceramic ball of such as silicon nitride has recently been investigated as to application. Ceramic ball has very high hardness in addition to preferable sliding property, and is least to receive injuries. Therefore, making a rolling element of the ceramic ball, abrasion by adhesion is suppressed, and it has been seen that fretting durability is far increased in comparison with steel ball. Further, a rolling element of ceramic ball is very excellent also in sound durability after continuous driving at high speed.
However, the ceramic ball is very expensive, and is considerably large in elastic coefficient comparing with steel. Therefore the bearing ring is easily formed with indentations by shock load and easy to invite sound deterioration. Besides, since the ceramic ball is very small in coefficient of linear expansion than the steel ball, it has also a problem that preload escaping is generated by rising of temperature when starting a machine, so that rigidity is decreased. As the ceramic is an insulator, static electricity collects foreigners to make rate of badness grow due to noises of dusts. Further, being very small in specific gravity, it has also another problem to easily cause transporting troubles by static electricity during setting up the rolling bearing apparatus.
Accordingly, the object of the invention is to solve various problems involved about conventional rolling bearings, and to provide such a rolling bearing which is excellent in fretting durability and impact resistance, less to create problems by preload escaping or static electricity (making rate of badness grow due to noises of dusts, easily causing transporting troubles by static electricity during setting up the rolling bearing apparatus), is excellent in sound property, and is low-cost.
To solve the above object, there is provided a rolling apparatus, wherein rolling elements are disposed between an outer member and an inner member, and the rolling elements are rollable with respect to a first contact face being a contact face of the outer member to the rolling elements and a second contact face being a contact face of the inner member to the rolling elements, wherein at least the rolling element is composed of a steel of Cr concentration Cr % being 8 wt % or higher, and an accomplished rolling element is provided on the surface thereof with a layer of N concentration N % being a range of 3xe2x89xa6N %  less than 0.26 xc3x97Cr %+4.42.
The rolling bearing of the invention is structured of a steel (base material) of the above mentioned composition, and a nitride layer having an appropriate nitrogen concentration and hardness is furnished on at least surface of the rolling element, and is therefore excellent in quietness, sound durability and fretting durability similar to hybrid bearings where the ceramic ball is used as the rolling element. In addition, the rolling bearing of the invention does not have shortcomings of the hybrid bearing, namely, it is good in impact resistance and difficult to cause problems by of preload escaping or static electricity, and it can be suitably used to machinery encountering vibrations as small-sized information machine, or demanded to have high quietness.
As the movable body, such as the outer race is taken up, and as the supporting body, the inner race or an axial body is taken up.
For a steel composing members of rolling bearings (movable body, supporting body and rolling elements), in view of making the surface hardness constantly Hv1100 or higher, and making surface characteristics, in particular, the fretting durability good, a steel containing Cr of at least 8 wt % is desirable. If the Cr concentration Cr % is less than 8 wt %, chromium nitride of an enough amount is not precipitated in the nitride layer, whereby the surface hardness is not often made Hv1100 or higher stably, and the sound durability and the fretting durability trend to be lower in comparison with steels of the chromium concentration of more than 8 wt %.
In contrast, if the chromium concentration is too high, it not only invites cost-up, but also generates a brittle xcex4 ferrite phase, and the chromium concentration is preferably 20 wt % or lower. Besides, if the chromium concentration is high, coarse eutectoid carbide exceeding length of 10 xcexcm is generated to cause processing troubles, and an aiming precision cannot be accomplished during finishing process. Therefore, desirable is such a steel where the relation between the carbon concentration C % and the chromium concentration Cr % contained in the steel satisfies C %xe2x89xa6xe2x88x920.05xc3x97Cr %+1.41. Other than chromium, if nitride forming elements such as Mo, V, Nb are added in complex, it is possible to more stabilize the surface hardness, and so those may be added in a range as far as cost permits.
Here, a description will be given in detail of the nitride layer of the present invention. FIGS. 7A and 7B are cross-sectional views showing a structure of a nitride treated member, wherein FIG. 7A is the member after the nitride treatment, and FIG. 7B is the member of the accomplished product.
After the nitriding treatment, on the surface of the member, as shown in FIG. 7A, the nitride layer is formed. This nitride layer is composed of a first layer and a second layer just under the first layer, the first layer comprising a false ceramic layer (nitride layer such as Fe2N (xcex6 phase), Fe2-3N (xcex5 phase), Fe4N (xcex3xe2x80x2 phase) and the second layer comprising much quantity of nitrides (Fe2-4N, CrN, Cr2N) and tempered martensite.
The false ceramic layer of the first layer is easy to be around 5 to 20 xcexcm in thickness at the treating temperature is high. This false layer is excellent in slidability and can be suitably used to various kinds of sliding members, but very brittle in comparison with the second layer. In the case of the rolling bearing receiving high shearing force, in particular when high quietness is requested, the existence of this layer is not preferable.
In the second layer containing the tempered martensite, the amount of nitride is much in a region near the first layer, and this amount becomes decreased as coming near to a core portion. Thus, even in the region of the second layer, nitride has a property relatively near the first layer in the vicinity of the first layer.
Therefore, it is preferable that the nitride layer of the invention is as follows.
The nitride layer of the invention is formed by a technique of forming the first layer as least as possible, and is a nitride layer in the surface of an accomplished product by grinding and lapping (see FIG. 7B). That is, by grinding and lapping, the first layer and a region near the first layer in the second layer are removed, and the accomplished surface is the nitride layer of the second layer.
In the nitride layer of the invention, nitrides of one or two kinds or more of xcex6 phase (Fe2N), xcex5 phase (Fe2-3N), xcex3xe2x80x2 phase (Fe4N), CrN, and Cr2N are precipitated, and the nitride layer includes a tempered martensite. Therefore, the hardness is considerably high, and in addition thereto, the nitride layer of the invention is also excellent in a toughness, so that durability of the material of the rolling bearing such as the rolling element is rapidly heightened and the sound deterioration caused by injuries in the members of the rolling element is remarkably suppressed. But, even if being the nitride layer in the second layer containing the tempered martensite, when the nitrogen concentration is too high, and C %xe2x89xa60.26 Cr %+4.42 is not satisfied, the tempered martensite of the base decreases, and brittleness much advances not to satisfy the sound durability of the rolling bearing and the durability of such as impact resistance. If the nitrogen concentration is less than 3%, the amount of precipitating nitrides is short and the surface hardness is lowered, and similarly the durability is decreased. From the above, the nitrogen concentration in the nitride layer is preferably in the range of 3%xe2x89xa6N %xe2x89xa60.26 Cr %+4.42.
The surface hardness is preferably above Hv1100. Being less than Hv1100, the nitride layer in the surface is easily injured, and bad influences may be given to sound characteristic or durability.
Also when the thickness of the nitride layer exceeds 2% of diameter Da of the rolling element, the impact resistance tends to go down. In the vicinity of the boundary between the nitride layer and the base material, the precipitating amount of nitride is inevitably short, so that sufficient hardness and durability are not available. So, it is preferable that the thickness of the nitride layer is 3 xcexcm or more, and 2% or lower of Da.
With respect to the steel (base material) composing the rotor and the supporter, when the rolling element satisfies the conditions of the invention, if the hardness is above HRC58, SUJ2 described in JIS G 4805 is of course, any steels for bearing are sufficient. But, if contained carbide is large or if an amount of residual austenite is much, satisfactory quietness or durability, for example, impact resistance cannot be obtained. Accordingly, it is preferable that the length of contained carbide is below 5 xcexcm and an amount of residual austenite is below 6 vol %. If the hardness of the steel composing the movable body and the supporting body is less than HRC58, there will occur problems that the fatigue strength is reduced, the fretting durability and the impact resistance are lowered.